1. Field of the Invention
The present invention relates to continuous plasma processing of materials, and more particularly to an apparatus and method for plasma processing which utilizes a gas injected in the center of a coalesced plasma column for obtaining desired heat transfer to the anode and/or particulate material processing.
2. Description of the Prior Art
Thermal plasmas, formed by arcs between a cathode and an anode, generate high temperatures and have an extremely active nature, and thus have attracted the interest of metallurgists and chemical synthesists for years. High intensity arcs are used for welding, cutting, plasma spraying, lighting, interrupting high power circuits, melting and alloying, and producing ultra fine refractories in the form of particulate material. It is known that arc induced magnetohydrodynamic effects dominate a high intensity arc. Interaction of the arc current with the self magnetic fields gives rise to a pumping action, inducing jets or gas flows called a cathode jet and an anode jet. The action of such induced gas flows depends on the gap between the cathode and anode and on the current strength, and can substantially affect the total heat transfer to the anode.
The present invention uses a multiple arc system, in the preferred form. In conjunction with the multiple arc system known electrical circuity is used as explained in an article by J. E. Harry and R. Knight entitled "Simultaneous Operation of Electric Arcs From The Same Supply", IEEE Trans Plasma Science, PS. 9(4) pp. 248-254 (1981), and also in an article "Power Supply Design For Multiple Discharge Arc Processes", published in the 6th International Symposium on Plasma Chemistry, Symposium Proceedings, Volume 1, pp. 150-155 (1983).
Yet, problems persist in establishing a stable, coalesced DC arc using multiple cathodes and/or plasma torches, that can be controlled to (1) permit injection of particulates into the plasma region for processing and permit ejection of these particles radially outwardly at a position spaced from the anode and (2) control anodal heating. (As used herein, "plasma torch" shall mean a device consisting of a thin stick-type cathode surrounded by a tube shaped anode with a nozzle at one end. When suitable electric power is supplied to a plasma torch, a jet of plasma is emitted from the opening of the nozzle.) Problems with heat dissipation over the surface of the anode also continue to exist. It is desirable to prevent localized extremely high temperature spots on the anode because hot spots cause detrimental evaporation of the anode material. Even when liquid cooled anodes are utilized, evaporation is a problem.
The Harry articles referred to above describe use of a plurality of cathodes spaced about a central axis for generating arcs in which plasma is formed, and U.S. Pat. No. 3,989,512, issued Nov. 2, 1976 to Sayce also describes a plurality of separate nontransferred arc plasma torches arranged in spaced relation around a central axis for generating a column of plasma.
Background on plasma technology is also given in the article entitled "Plasma Technology And Its Application To Extractive Metallurgy" by S. M. L. Hamblyn, Mineral Science Engineering, SCI. Engng, Vol. 9, No. 3, pp. 151-176 (July 1977).
On Page 153 of the Hamblyn article, in FIGS. 6 and 7, there is a three phase (AC) plasma system stablized by using a direct current supply. The arrangement forms a central plasma stream, and it is indicated that the device had a central gas flow along the plasma stream, to produce a homogeneous volume of plasma. The system was used mainly for experimental studies of heat transfer to refractory oxide in a fluidized bed reactor. The device in this article does not provide for a coalesced DC arc, such as is obtained with the three cathodes utilized with the present device, nor does it teach the use of a gas stream for stablizing the arcs, and/or material particle injection and processing.
Additional work is set forth in the article of C. Sheer et al entitled "Invited Review: Development And Application Of The High Intensity Convective Electrical Arc", Chem. Eng. Comm. Vol. 19, pp. 1-47 (1982), and in particular pages 17-27 include an explanation of a "cathode pump" that forms near a cathode tip. A single cathode is disclosed and in the article it is taught that particulate matter being introduced into the plasma is to some extent injected into the plasma, but for the most part travels in the fringes of the plasma, and thus the high temperature plasma core which is useful for processing is not fully utilized.
An additional patent which illustrates the general state of the art in plasma arc production of silicon nitride is U.S. Pat. No. 4,206,190, issued June 3, 1980 to Harvey et al. This describes a plasma arc furnace used for forming particulate silicon nitride.
Arc plasma dissociation of zircon is described in an article by that title in Chemical Engineering, Nov. 24, 1975. This just shows general applications of plasma technology.
Additional patents which illustrate use of plasma arcs and/or plasma generators are as follows:
______________________________________ U.S. Pat. No. Inventor Date of Issue ______________________________________ 3,313,908 R. Unger et al 04-11-67 3,496,280 D. Dukelow et al 02-17-70 3,573,090 J. Peterson 03-30-71 3,980,802 B. Paton et al 09-14-76 4,121,083 R. Smyth 10-17-78 4,141,694 S. Camacho 02-27-79 4,426,709 J. Fegerl et al 01-17-84 ______________________________________
Although multiple cathode plasma generators are known in the prior art, several problems persist: (1) lack of stability of the plasma column and a tendency for the column to randomly dance across the anode surface; (2) localized hot spots on the anode surface that cause unwanted evaporation of the anode; (3) maintaining high heat flows to the anode without unwanted hot spots; (4) difficulty in overcoming the viscosity and thermal gradient effects of the plasma to inject particulate matter into the plasma for processing; and (5) maintaining particulate matter within the plasma for sufficient times to ensure efficient and complete processing. It is to the solution of these problems that this invention is directed.